National Rail Conditions of Carriage, Condition 13, permits customers to travel by a different route, from the one stated on the ticket or allowed by conditions of the National Routeing Guide, to make their journey on payment of an Excess fare. The Excess fare is calculated as follows:
Single tickets
The difference between the price already paid and price of the cheapest Single ticket, available for immediate travel, that allows the customer to travel on the route and Train Company of their choice
Return tickets - change of route in both directions
The difference between the price already paid and price of the cheapest Return ticket, available for immediate travel, that allows the customer to travel on the route and Train Company of their choice.
Return tickets - change of route in one direction only
Half the difference between the price already paid and price of the cheapest Return ticket, available for immediate travel that allows the customer to travel on the route and Train Company of their choice.